High pressure laminates, either decorative or industrial, are composite materials--tough cellulose fibers embedded in a matrix of brittle resin. The resulting material possesses properties different and frequently better than either component. High pressure laminates, for example, possess considerably more flexibility than the cured resin and more water resistance than the fiber.
Westvaco Corporation is a major supplier of resin saturable paper (also known as saturating kraft paper) for laminate formation. This conventional resin saturable paper is formed from a blend of hardwood pulp fibers and softwood (pine) pulp fibers, each of which type fibers are liberated from wood chip via the kraft pulping process. Prior to resin saturable paper formation, the pulps are subjected to low consistency (approximately 3%) refining.
The resin saturable paper is immersed in a bath of resin solution, and excess resin is removed from the surface of the web by squeeze rolls or scraper bars. The sheet is passed through an oven to evaporate the solvent in the resin to a level of 6-8% volatiles. The web is then cooled and either wound in rolls or sheeted to size. Resin-treated sheets are laid up to the desired number of plies and then consolidated under heat (ca. 300.degree. F.) and pressure (ca. 1000 psi). During this operation the resin flows sufficiently to displace air between the plies. Simultaneously the resin polymerizes into a rigid solid. The result is a monolithic structure, the finished composite.
To perform satisfactorily in this service, a saturable paper must possess a special combination of carefully-controlled properties. First of all, the basis weight must be controlled within tight specifications. Not only must it be controlled across and throughout a roll, it must also be controlled on a quarter-inch to two-inch scale. This latter property is generally referred to as formation and is judged by the show-through of light through the sheet. In this case, thin places transmit more light than the heavier weight regions. For a good saturable paper, this formation or show-through should be of low contrast with little difference in light transmission from the darkest to the lightest places.
Good saturable sheets are also relatively clean without sizeable shives or unfiberized pieces of wood. Such material constitutes non-uniformities in the structure causing surface roughness and points of stress concentration. This material is not readily impregnated with resin and thus can become the site of blister initiation.
The most important properties of saturable papers, however, are those that control the rate of resin imbibition and its distribution throughout the sheet. These two distinct physical processes take place simultaneously and consecutively. Both processes are essential to the manufacture of satisfactory composites.
The first process is called saturation. It involves the pickup of resin by the porous structure of the web. It begins when the web enters the resin bath and ends when the scraper bars or other devices remove the excess resin. This process determines the ratio of resin to fiber in the final structure. In general, sufficient resin must be used so that all voids in the product are filled. At the same time, resin is more expensive than paper, so economics dictate against the use of excess resin. Finished laminate properties also begin to suffer if excessive quantities of resin are used.
In general the saturation of the paper is controlled by the pore structure of the paper, the viscosity and surface tension of the resin, and the time required to travel from entering the resin bath to the scraper bars. In practice, the major control is the structure of the paper. This must be tailored to the rest of the operation so resin pickup will be at the desired value. Resin properties and speed are fine-tuning controls.
Once the proper amount of resin has been incorporated into the web, the next concern is achieving uniform and complete distribution of the resin throughout the web. This involves the second of these two distinct physical processes--penetration. It, too, is extremely dependent on the structure of the web. Capillary forces in the pores of the sheet act on the resin solution to redistribute the resin. Fine pores will steal resin from the large pores. The total amount of resin in the sheet becomes an important variable since this determines the quantity of resin to be shared by the various sized pores. In practice, an excess of resin is used to be sure there are no voids where the resin has not reached. As pointed out above, this excess is uneconomic and therefore should be minimized.
Studies of the pore size distribution of paper used in high pressure laminate manufacture indicate this is an important variable. The effects on saturation and penetration, however, are quite different. Saturation is a short time process--on the order of a second. It involves the time between applying the resin and the removal of the excess. During this short interval, most of the resin is picked up in the larger diameter pores, i.e., those 10 micrometers and above in diameter. The smaller ones are also picking up resin but the dynamics favor the larger pores. To enhance saturation, large pores are needed.
Penetration, on the other hand, is a longer term process which starts with the initial contact with the resin and probably does not come to a halt until the resin is completely polymerized in the press. During penetration, the smaller pores or capillaries are stealing resin from the larger pores. It is this process that spreads the resin from the surfaces that contact the resin to the interior of the sheet. Without good penetration, white centers are observed in the saturated sheet. This white is dry fiber which has not been wetted with resin. In general, penetration is enhanced by any process that increases the proportion of pore volume that exists in the smaller pores. This obviously tends to reduce saturation so, in practice, a balance must be maintained. As mentioned earlier, excess resin is used to insure excellent penetration, otherwise voids occur which reduce strength and water resistance and which may become loci for blister formation.
Paper is a network of crossing fibers, more or less bonded to each other. Loose ends from some of these fibers project above the surface of the paper. As the paper proceeds through different machinery some of these fibers are pulled free, producing what is known in the industry as fuzz, dust, or lint.
Saturating kraft is a special type of absorbent paper designed to be impregnated with resin and, therefore, is used primarily for the core stock of laminates. Fuzz released from saturating kraft has traditionally been a major problem for laminate producers. During the production of laminates, saturating kraft paper passes through a phenolic resin bath which makes the paper extremely sticky. Since loose fuzz has a tendency to agglomerate and form fuzz balls, it is common for some of these fuzz balls to collect on and stick to the resin-treated paper. As the paper proceeds through the drying process and becomes cut, sandwiched, and pressed into finished laminates, adhering fuzz balls can cause surface imperfections known as pressure marks. Because pressure-marked laminates are unsuitable for commercial use, these fuzz ball-induced imperfections are an important cause for the rejection of laminates.
Various methods have been tried to address this fuzz problem, including varying the composition and refining level of the paper pulp and vacuuming the paper prior to, and during, laminate production. Although each of these attempts have met with varying degrees of success, pressure marks are still a major problem for the laminate industry.
Therefore, it is the object of this invention to provide an effective and economical method for reducing pressure marks in the production of laminates. Other objects, features, and advantages will be evident from the following disclosures.